Process for sealing cartons



y 7, 1964 'T. w. RUSSELL 3,140,215

PROCESS FOR SEALING CARTONS Filed Dec. 16, 1959 Invenlor Theron ZdfizsgeZZ United States Patent 3,140,215 PROCESS FOR SEALING CARTONS Theron W. Russell, Skokie, Ill., assignor to Weyerhaeuser Company, Tacoma, Wash, a corporation of Washington Filed Dec. 16, 1959, Ser. No. 860,024 9 Claims. (Cl. 156-309) The present invention relates to generally sealing fiberboard together and to sealing fiberboard cartons, with special reference to the provision of sealed cartons for deep-frozen foods.

In the carton field, difiiculty has been experienced in sealing Wax-coated cartons so that they remain sealed when used for deep-frozen foods. One difficulty heretofore encountered, especially with heat-sealed flaps, is that the flaps snap apart at very cold temperatures.

Wax-coats, or coats of other moisture-barrier material 'are used on cartons for the storage of food to prevent the ingress and egress of moisture vapor. Not only does the presence of the wax introduce a problem of sealing at deep-freeze temperatures, but the carton-stock itself has presented problems in the matter of resistance to transmitting moisture vapor. It has been discovered that without certain characteristics in the fiberboard, the folds therein necessary for carton formation readily crack at deep-freeze temperatures when such a cold carton is subjected to undue stress or mechanical shock, as in handling, shipping and transferring supplies for deep-frozen filled cartons.

In the frozen food field, the pack delivered to a consumer presents printed matter. The presence of wax or the like on the fiberboard stock of a carton makes it difficult to impossible to print on the carton, so the carton is commonly made of unprinted stock, waxed, and when sealed is wrapped in a thin printed paper which is also waxed for providing the desired degree of resistance to transmission of moisture. Such paper develops wrinkles which do not lead to a complete seal. This is unattractive for displaying merchandise. In the hands of the consumer, the paper wrapping must be torn open and usually separates from the carton. Frequently, this results in destroying identity of the source of the goods and the ready availability of the directions for consumer use, which are printed on the paper.

Wax-coating on carton stock also interferes with sealing the carton flaps together to make a tight moisture-retaining package. Heat-sealing is practiced, and likewise adhesive compositions are employed over the Waxed surfaces. The packages so sealed have cohered flaps, that is, one flap sealed to another flap by an interfacial layer, which may be the wax or wax with other adhesive. Such cohesive union does not withstand very low temperatures, such as 50 F. The interfacial layer yields and the flaps spring apart, especially when handled at such low temperatures.

When waxed cartons are heat-sealed, it is common for the applied heat to be conducted beyond the region of the flaps to an extent melting wax on a carton wall and at an edge, resulting in a contrast with the inner portion where wax is not so melted. Such contrast is designated as burn. Sometimes the burn involves bubble-formations in the melted wax resulting from release of water vapor from the carbon stock by the conducted heat. This burn is partially hidden by the waxed paper wrapper, and is a problem respecting attractive appearance for display.

According to one feature of the present invention, the waxed paper covering may be dispensed with, and the cartons may be printed before waxing, thus eliminating the paper-wrapping and sealing operations and presenting a neater package, and one more useful to the housewife.

According to one feature of the present invention not pertaining necessarily to cartons, uncoated fiber- .of these conditions.

3,140,215 Patented July 7, 1964 board stock is coated by Wax or a like fusible coat and has a body absorptive of the coat when fused, thus to expose the fibers, so that two such sheets may be bonded with a fiber-to-fiber bond.

According to another feature of the present invention, cartons of waxed fiberboard are sealed flap-to-flap by fibers of one flap bonded to fibers of the adjacent flap, and such flaps may be sealed so as to maintain the seal at 50 F., and even at lower temperatures.

According to another feature of the present invention, that heating for sealing which commonly effects burn, is promptly followed by chilling to avoid burn, which if it should occur would be more conspicuous in unwrapped cartons than burn in paper-wrapped cartons.

In carrying out the present invention for the more rigid requirements of cartons, the characters of the carton stock, of the wax coat, and of the adhesive are controlled for a processing which produces sealed cartons with desirable properties in numerous stages, such as manufacture of carton blanks, shipping of fiat-folded cartons, setting up and filling cartons, sealing cartons for deepfreezing, retaining and rejecting moisture during and after deep-freezing, and thereafter, for the display of frozen packs, and utilization thereof by the consumer.

Carton stock is commonly provided as Fourdriniermade fiberboard. For the bonding requirements of the present invention it should be absorptive of the melted form of a fusible moisture-barrier coat material to be applied. For deep-freeze requirements of the present invention it should be flexible for bending without rupture, and when bent to corners, the corners should be stable against rupture upon mechanical movement or shock at deep-freeze temperatures.

It has been found that fiberboard made from bleached sulfate fibers which ordinarily has a pH of 4.3, when made into a carton for heat-sealing for subsequent storage under deep-freeze temperatures, does not meet the requirements It becomes brittle and unstable at carton folds and otherwise does not have adequate resistance to mechanical shock. However, these inadequacies can be corrected and the requirements imposed by the aforesaid conditions can be met by making or adjusting the pH of the fiberboard at a value in the range from 5.2 to 5.7, and preferably 5.5 to 5.7.

It is conventional practice to print on flat stock to be die-cut and scored into flat carton blanks, and to facili' tate the printing, it is conventional to coat or size the carton stock, as with clay. For the present invention, the portions of the stock to be adhesively united fiber-tofiber in sealing the flaps of the carton should not be covered by clay or by ink. Practically, this means that the initial supply of carton stock should not be so coated overall as to cover the fibers. However, a small amount of coating material, such as clay, may be used to level the microscopic irregularities in the surface, so long as fibers remain exposed, not only to drink in the fused barrier-coat to the absorptive body, but also to function for the fiber-to-fiber adhesion. Leveling for receipt of printing may be enhanced by calendering.

A suitable fiberboard is a so-called paperboard felted on a Fourdrinier machine from a furnish containing 1000 parts (dry basis) of bleached sulfate cellulose fibers such as those derived from Douglas fir, 5 parts of clay, 2.5 parts of gelatinized starch, and a bufier, such as sodium alnminate, or proportioned quantities of alum and caustic soda, to provide a pH in the range 5 .2 to 5.7, and preferably, 5.5 to 5.7. Small quantities of Wax or rosin may be present to impart water resistance, where such is desirable.

The Wet pressed sheet is surface-sized on both faces with an aqueous size containing clay and dispersed wax with binder of starch or starch derivative. This is applied to add about 0.42 pound (dry basis) per 1000 sq. ft., of each face, to provide filler but not a coat, to facilitate printing and to prime the surface to minimize penetration of the waxy material in cold-water waxing.

Further, to improve the surface for printing, each face is calendered using an aqueous solution of a starch or starch derivative and sodium alginate, adding only about 0.19 to 0.22 pound of solids to each face per 1000 sq. ft.

The finished stock has a moisture content of 4% to 6.5%, and for the general run of cartons has a weight of 195 pounds per 500 sheets 24 x 36 inches, further identified as 15-point full-bleached uncoated kraft board.

The pH is important where bending and moisture-retention of scaled cartons is desired. This is exemplified by comparing stocks of pH 4.3 and 5.5. Strips of the stock 15 mm. wide were variously tempered and tested for bend with a fold tester, specifically that one known as Massachusetts Institute of Technology Folding Endurance Tester, manufactured by Tinius Olsen Testing Machine Company, Philadelphia, Pa. The test results designate the number of times a strip may be bent around a small cylindrical surface and back again in the opposite direction until it ruptures, as detailed on pages 864- 866 Testing of Paper and Board, first edition, 1953, Me- Grew-Hill Book Company, Inc.

In Test A, the strips were heated at 105 C. for 72 hours, and then tested at room temperature of approximately 73 F. In Test B, the strips were heated at 260 F. for hours, then cooled at F. for 16 hours, and then tested when cold at 10 F giving results as follows:

Table 1 D A I B 4. 3 2 to 3 1 to 26 5. 5 109 101 So, partially coated, or not at all coated, the stock commonly in multiple-blank sheets, may be printed in a joined sequence of impressions and then die-cut and scored to individual foldable blanks. However, the layout of printing must be such that the flap areas to be sealed are free of printing ink as well as free of a cover coat of clay. Then, the printed stock is cut to individual flat blanks.

The next step for deep-freeze cartons is to coat the fiberboard with fusible moisture-barrier material. Natural and artificial waxes and wax-like materials are available for the purpose, those commonly used being selected from the group consisting of paraffin wax, microcrystalline wax, polyethylene Wax and mixtures of these Waxes. Parafiin wax is preferred to provide a transparent coat when there is printed matter to be visible through the coat.

When such hydrocarbon wax is used the method of application is such as to minimize absorption by the fiberboard, so that for subsequent operations within the scope of this invention there is capacity to absorb the coating material when it is fused. The preferred method of application is that known as cold-water waxing.

In cold-water waxing the fiat carton blanks are passed individually by immersion through a bath of a melted waxing material, and quickly removed to suitable rolls which remove some of the excess at the surface, preferably by squeezing, leaving a total of about 5 to 6 lbs. per M sq. ft. The hot waxed stock is then passed immediately through a cold water bath, chilling the molten wax and providing a continuous coat of wax on the two surfaces of a remaining absorptive body, and on all of the cut edges.

The preferred Waxing composition is essentially a paraffin wax, With added material for the present invention. The waxed stock must be flexible to bend at the folding lines without cracking the wax coat. It must be such that blocking or sticking together of a stack of flattened carquired for use of hot melts and the like.

tons is avoided. For these results, a blend of polyethylene wax and paraffin wax is conventional, as set forth in US. Patent No. 2,892,735. In addition, it imparts gloss and slip to the package, and grease-resistance. However, the polyethylene-paraffin coat has the disadvantage that it resists the action of adhesives, and interfacial adhesives used on such waxed surfaces form a poor interfacial cohesive bond which springs open at deep-freeze temperatures.

A composition of 97.5 parts of paraffin (M.P. 139 F.) and about 2.5 parts of polyethylene (M.P. 189 F., molecular weight 13,000) is suitable for the advantages above set forth. It is applied in melted form, preferably at 200 F. to 210 F., or otherwise below the boiling point of water, thus minimizing absorption and avoiding bubbles in a wax coat by release of moisture.

After the flat carton blank is fully waxed it is converted to fiat blank foldable form. In the case of sleeve-type cartons, one overlap seam is provided, preferably at a narrow face of the carton. The conventional procedure is to pass the flat carton blank through a dewaxer, in which a blast of hot air melts and blows away the wax at areas to be united. Thus, the fibers are exposed, and any suitable adhesive used to unite the faces. Preferably, an adhesive dissolved in a volatile solvent is employed, because at this point there is no danger of residual solvent being retained at the time food may be entered into the carton for sealing.

The present invention effects a fiber-to-fiber bond between ilaps by placing the flaps in facial contact at the positions to be united, melting the wax on the flaps and absorbing it at the areas to be united and in the presence of a fluid adhesive composition immiscible with the solid and the molten wax. The absorption of wax exposes the fibers in adjacent flaps to adhesive composition which is activated by the effect of heat to bond the fibers of one flap to the fibers of the contacting flap. After adhesion, the sealed flaps are immediately cooled when avoidance of burn is desired.

The adhesive composition is applied over the solid wax coat in metered quantity and in liquid form facilitating its application. The fluid adhesive composition is applied at the temperature of the machine environment, thus eliminating the means and processing heretofore re- This permits better control of the application and avoids spot effects frequently encountered with hot applications at this stage. Adhesive composition is so compounded that it wets the paraffin coat at least to a degree assuring an adequate supply. A creamy viscous consistency readily clings to the waxy coat. The adhesive in the final bond is preferably a resinous composition, suitably plasticized when required, to withstand very cold temperatures so that it does not embrittle and crack, and to impart moisturebarrier properties. When the wax coat melts in contact with the liquid adhesive, the melted wax acts as a flux which aids in distributing or leveling the applied composition, Which then becomes the active binder after the surface fibers have been exposed.

The preferred adhesive compositions have a water base serving as solvent or vehicle. Starch pastes or protein glues are useful. It may be an aquasol, or aqueous dispersion containing resinous binder material such as dispersed particles such as polyvinyl acetate and a plasticizer therefor, which particles coalesce to form a bond as the composition loses water at a temperature above the melting point of the wax. The loss of water may be in part due to absorption by the fiberboard and in part to boiling it away by the applied heat. The aqueous resinous dispersions are such that a thin film as it dries forms an elastic rubber-like film by reason of the dispersion particles uniting. In the present invention, after the wax coat is removed from the fibers by melting and absorbing it, the fibers of adjacent flaps are in contact in the presence of the dispersed particles which also enter inter-fiber spaces. So positioned, they coagulate by the effect of heat and loss of water forming a solid elastomeric body including film coating enclosing fibers and connecting fibers of one flap to fibers of the adjacent flap. An aqueous dispersion permits control of viscosity for application, and the aqueous phase provides a vehicle for a surfactant or dispersant which maintains the dispersion and which is a suitable wetting agent, preferably nonionic, to insure contact and retention by the solid wax coat of a layer or streak of dispersion. The applied adhesive composition is metered in application, and metering is preferably accomplished by applying parallel streaks of adhesive, the contents of which flow and spread out in a layer as pressure is applied to contacting flaps. Application of the adhesive to but one of any two flaps to be united is sufficient.

For sealing-in of moisture, a carton must have a full quota of sealing material inwardly from the edges of flap interfaces which are exposed to the atmosphere. In the case of a rectangular carton with its four walls extended as flaps from hinging lines with their walls, two opposite flaps are first folded in as inner flaps. Then, an intermediate flap is folded over the inner flaps, and lastly a fourth flap is folded to overlap the intermediate flap.

Adhesive may be applied in a variety of areas. The outer faces of the inner flaps must unite with the inner face of the intermediate flap. The inner face of the fourth flap must unite with the outer face of the intermediate flap. The outward extents of the inner flaps from their hinging lines are usually not longer, and preferably much shorter than half the length of the hinging lines of the remaining flaps. For a perfect moisture seal, the inner flaps should be sealed over the entire lengths of their hinging lines and inwardly therefrom, but not necessarily over the entire face of the inner flaps. Acco-rding ly, it is preferred to apply adhesive only at the end portions of the inner face of the intermediate flap where it will transfer on folding to the inner flaps. Where the adhesive is applied in streaks, as mentioned above, means is provided to assure flow of adhesive to the ends of the hinging lines of the inner flap and over a complete sealing area. One such means is a score-line lying within outer faces of the inner flaps, each score-line being continuous from one end of the hinging line to the other end. A score-line shaped as a V from end to end of the hinging line provides a channel of the score-line from corner to corner when the flaps are pressed together.

The said score-line has another important function in rigidifying the inner flaps from their hinging lines inwardly, so as to exert resistance when the flaps are moved inwardly to final position by mechanism on a closing machine or by the intermediate flap and the fourth flap. The rigidifying effect maintains the portion between the hinge-line and the score-line as a planar portion for a flat area of fiber-to-fiber bond.

When the fourth flap completely overlies the intermediate flap it is most convenient mechanically to apply adhesive composition to the outer face of the intermediate flap in amount completely to seal the interface. However, when the fourth flap overlaps only a portion of the intermediate flap the adhesive composition may be applied to the inner face of the fourth flap, or otherwise to the outer face of the intermediate flap but on an area limited to supply the interface only. After applying the adhesive composition, the flaps may be folded by conventional means into the proper contact relationship of final position, and so held until united. In this position, no action takes place until the temperature at the interfaces is raised, first to melt the wax to provide a flux assisting the distribution of the liquid composition, then to absorb the wax into the fiberboard at the areas to be united, and then to effect coagulation of the adhesive solids of the composition. Heat may be applied by dielectric means eifective because of the moisture present, or otherwise, for

example, by contact of the closed flap with heated platens. In a continuous operation, the closed end may slide over a series of heated platens on which the temperatures for heating and for subsequent cooling may be regulated according to variables in the process.

When heated platens are used in a continuous processing they are preferably divided into sections of different temperatures, higher at the first contact and then decreasing. The rate of travel on a conveyor, the dimensions of the end contacting the heated plates, the thickness of the carton stock, the melting point of the wax coat, and the coagulation temperature of the dispersion, all predetermine the time of contact and the lengths and temperatures of the several heated zones of contact. As one example, the plates have zones heated at 375 F., 325 F., 275 F. and 225 F., followed immediately by cooling plates having a temperature of 30 F.

FIG. 1 in the drawing is a perspective View of a sleevetype carton as set up from fiat-folded form.

FIG. 2 represents diagrammatically two rolls for applying adhesive.

Numerals 10, 11, 12 and 13 represent the four side walls of a rectangular carton, of which side Wall 13 contains an overlapping glued seam at which one edge 14 of the flat blank is indicated at the interior of the carton. The junction line 15 between the faces 12 and 13 defines with said edge 14 a narrow strip of the blank which extends as a tab 16 beyond the opening of the carton defined by said four side walls.

Numeral 17 is a graphical representation of contents to be sealed within the carton. The side walls 10, 11, 12 and 13 are extended as flaps 20, 21, 22 and 23, respectively, with hinging lines respectively designated 24, 25, 26 and 27 defining the opening of the carton to be closed by folding the flaps. Flaps 21 and 23 are inner flaps. Flaps 22 and 24 are respectively intermediate and outer flaps.

When the flat carton blank is set up to rectangular form, the flaps are in the position indicated, except that the flap 20 may be folded up as shown in dotted lines rather than coplanar with its side wall 11), depending upon whether or not clearance is needed for means used for applying adhesive to flap 22. The preferred manner of applying adhesive is to apply it in metered streaks 30 and 31 to the inner face of flap 22 and overall at the outer face of flap 22, this being indicated by an opening 32 in flap 22 merely to illusrate adhesive 33 on the outer side of flap 22. The extent of the streaks 30 and 31 inwardly from the ends of flap 22 is substantially the same as the extent of flaps 21 and 23 away from the opening.

On the outerfaces of said inner flaps 21 and 23, there are score-lines designated 35 and 36 extending from end to end of the respective folding lines 25 and 27. These score-lines rigidify the said inner fiaps for an extent away from their folding lines and provide between each scoreline and the folding line an area remaining planar as the flap is moved inwardly to scaling position. In the case of the inner flap 23, said tab 16 is adhesively united to it and provides additional resistance to exert slight pressure against intermediate flap 22 when the latter is folded onto the inner flaps.

In commercial practice, a succession of cartons in the form illustrated, but without the adhesive shown, is conveyed rapidly in the direction of the arrow 40. As the illustrated carton advances, it passes conventional plow means (not shown) which turns inner flap 21 into sealing position, and passes conventional tucker means (not 7 shown), which moves inner flap 23 into sealing position, I and then it passes through means for applying adhesive.

In FIG. 2, a driven cylindrical metering roll is illustrated for applying adhesive in said streaks 30 and 31. The roll 41 has spaced peripheral grooves 42 therein which are filled with liquid adhesive for transfer by contact to the flap 22. The dotted line position 43 of the roll 41 indicates a mechanically raised position of the roll which is effected as the area of flap 22 between the illustrated streaks 30 and 31 passes the roll. At the same time, a driven roll applicator 44 beneath the roll 41 applies adhesive 33 to the entire outer face of flap 22 in a conventional manner. Roll 44 dips in a supply 45 of adhesive liquid in a container 46. In use, the rolls 41 and 44 are normally in contact so that roll 44 supplies liquid to roll 41. As cartons advance past the gluing rolls, the flap 22 first enters the nip between the rolls. As the nonglued center portion of flap 22 passes the line of tangency cam means, indicated graphically by numeral 46, functions by means indicated graphically at 48 to raise roll 41 to dotted line position 43.

After the adhesive is applied, the flap 22 meets conventional means (not shown), which folds it up in FIG. 1 over the infolded inner flaps 21 and 23, and then similar means folds outer flap 20 onto the adhesive 33 on the infolded intermediate flap 22.

Thus, the flaps are in sealing position with the faces thereof to be sealed separated by an interfacial layer of liquid adhesive between wax coats. As the carton progresses, the end or ends having the folded flaps slide along heated platens which effect the final sealing as described, and then slide along cooling platens to remove unwanted heat when burn is to be avoided.

Usually, the sleeve-type carton has its two ends originally open and in such cases both ends are treated and sealed at the same time. Consequently, only one end of a carton is illustrated, it being understood that the operations may be duplicated at the other end.

In the showing of FIG. 1, the space between the streaks 30 and 31 is originally glossy as a result of the wax coat. This area may be used as a test for the proper application of heat. When this area has changed from a glossy surface to a matt surface, it is proof that the applied heat has effectively melted and absorbed the wax at this area within the sealed carton, thus indicating that all the areas outwardly therefrom have been properly heated for the same result.

Aside from aqueous starch and flour pastes, animal glue, and the like, there are many formulations of aqueous adhesives suitable for the present invention. For the more drastic requirements heretofore discussed, the preferred ones involve natural or synthetic resinous materials with plasticizer, when necessary, for imparting flexibility at normal and depressed temperatures. At the dilutions applied in the present invention, they coagulate or deposit a flexible elastic film on a smooth surface, either by loss of water or by the effect of elevated temperature. For the present invention, the temperature imposed on the carton for melting and absorbing the barrier coat is above the boiling point of water, and the adhesive is accordingly formulated to remain inactive until the material of the barrier coat is melted and absorbed. By using a for-mulation which becomes active by loss of water resulting from the imposed temperature, the desired result is assured.

Latex dispersions which would coagulate by heat prior to absorption of the wax may have the coagulation temperature elevated by adding a surfactant as described in U.S. Patents Nos. 2,912,349 and 2,912,350. Said patents describe numerous latices stabilized with anionic surfactant, or cationic surfactant, each with or without nonionic surfactant, or with nonionic surfactant alone. In general, increase in content of the stabilizing type or mixture of surfactant elevates the temperature at which coagulation occurs. Cationically stabilized latices, although useful, are in general undesired because they are substantive to many surfaces and deposit solids on contact. Nonionic and anionic surfactants alone or mixed, are accordingly the preferred surfactants, but it is to be understood that other agents may be used as dispersants or so-called emulsifiers. Surfactants assist in wetting the Waxy coat, but may be dispensed with where the viscosity of the aqueous adhesive is high, not only to prevent it running 8 freely, but also to minimize water content and thereby the duration of heating.

Among suitable latices are the following:

Dow Latex 546-0.

Butadiene-styrene copolymer.

48% solids, stabilized with anionic surfactant. L-Z:

Nitrex 2614.

Butadiene-acrylonitrile.

38.4% solids, stabilized with anionic surfactant. L-3:

Chemigum 235 AHS.

Butadiene-acrylonitrile.

43.4% solids, stabilized with ammonium soap as anionic surfactant.

6E3 Buna N latex.

Butadiene-acrylonitrile.

42% solids, stabilized with anionic surfactant. L-5:

Natural rubber latex.

59.2% solids, naturally anionically stabilized.

There are many surfactants commercially available, one or more of which may be present in the above latices, and which may be added to them to elevate the coagulation temperature. In the following exemplary list of surfactants, N designates nonionic and A designates anionic:

Brij 35. Polyoxyethylene lauryl alcohol. Liquid. N-2:

Nonic 218. Polyethylene glycol-tertiary-dodecylthioether. Liquid. N-3:

Triton X-100. Alkyl-aryl polyether alcohol. Liquid. N4:

Igepal CO. Alkyl phenoxy polyoxyethylene ethanol. Liquid. A1:

N.S.A.E. Sodium alkyl naphthalene sulfonate. 85% activepowder. A-2:

Nekal BX-78. Sodium alkyl-naphthalene sulfonate. activepowder. A-3:

Blankol N. Sodium salt of sulfonated naphthalene condensate. 80% activepowder.

For sealing packages containing food, the adhesive composition preferably contains ingredients meeting requirements imposed by governmental bodies, such as those which are colorless, odorless and non-toxic. Absence of certain volatile solvents is thus dictated, making Water the preferred sole liquid vehicle. For such use, polyvinyl acetate is a presently preferred resin base, with which numerous plasticizers may be employed. It can provide a firm nonfriable bond stable at 40 F., and at lower temperatures.

The preferred polyvinyl acetate is one of which from 25% to 50% is soluble in benzol at room temperature. Its aqueous plasticized dispersions having a viscosity in the range from 1500 to 2500 centipoises are suitable for application over the described wax coat, the viscosity rather than the solids content being a convenient control II for operation, and the solids content being a convenient control for viscosity.

Among the plasticizers available are: dimethyl phthalate, diethylphthalate, tributyl-acetyl citrate, butylphthalylbutylglycollate, dibutyl phthalate, dibutoxyethyl phthalate, methyl phthalyl ethyl glycollate, ethyl, phthalyl ethyl glycollate, triethylene glycol dihexoate, dipropylene glycol dibenzoate, triethyl citrate, tributyl citrate, and acetyltributyl citrate.

It has been found that a polyvinyl acetate dispersion made with polyvinyl alcohol as dispersing agent and having a heterogeneous particle size of approximately 1 to 9 microns, and a solids content of 55%, is an excellent base, designated below as PVAo55, for forming suitable adhesive compositions for food containers as follows.

Parts by weight PVAc-SS 100 Diethylphthalate Solids content, 57%.

PVAc-55 "100 Tributyl-acetate citrate 8 Water 2 Solids content, 57%.

PVAc-55 100 Butyl-phthalylbutylglycollate 12 Water 5 Solids content, 57%.

The above compositions are acceptable for food containers and have a suitable viscosity for use as formulated. If surfactant is desired, from 0.1 to 0.2% of anionic or nonionic surfactant may be present. It is to be understood that these compositions are creamythick and viscous, and may be thinned by adding water. If thinned and thereby rendered too low in viscosity, thickening agents may be added to raise the viscosity, such as methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose polyvinyl alcohol, gum guar, sodium alginate, and the like. However, such dilution is undesired Where the adhesive is used for deep-frozen food cartons.

The various general and specific aspects of the present invention are set forth in the appended claims without any intention to limit the invention to the illustrative details included in the foregoing specification.

I claim:

1. The method comprising converting a fiberboard sheet to a carton to be closed and sealed by flaps at a carton opening, said fiberboard having a pH in the range of from about 5.2 to about 5.7 and having an absorptive body, and said carton having flaps with faces in which fibers are exposed, coating the entire carton with a hydrophobic coat of normally solid fusible moisture-barrier material having a melting point at an elevated temperature below the boiling point of water, applying an aque ous adhesive composition activatable to adhesiveness by loss of water therefrom to said flaps whereby said adhesive composition will form an interface between pairs of said flaps when said flaps are closed, closing said flaps to scaling position, heating the so-positioned flaps by applying heat at a temperature above the boiling point of water and thereby melting and absorbing substantially all of the moisture-barrier material and exposing fibers of the flaps to the interfacial aqueous adhesive material, and thereby removing water from the adhesive material and bonding flap-to-fiap by a fiber-to-fiber bond.

2. The method of claim 1 in which the moisture-barrier material forms a transparent coat consisting of a major proportion of paraffin wax and a small proportion of polyethylene wax.

3. The method of claim 2 in which said adhesive composition contains a dispersed adhesive resinous composition.

4. The method according to claim 3 in which the dispersed adhesive resinous composition comprises polyvinyl acetate.

5. The method according to claim 3 in which the dispersed adhesive resinous composition comprises polyvinyl acetate and plasticizer therefor.

6. The method according to claim 3 in which the dispersed adhesive resinous composition comprises polyvinyl acetate and contains a plasticizer therefor and a wetting agent.

7. The process of claim 3 followed immediately after adhesively uniting the flaps, by cooling the sealed flaps and thereby drawing heat therefrom and minimizing conductance of heat inwardly of the carton from said heated sealed opening and minimizing possible alteration by the effect of heat of the appearance of the coated fiberboard.

8. The method of claim 1 in which the aqueous adhesive composition is an aqueous dispersion of an adhesive resinous composition.

9. The method of claim 1 in which the aqueous adhesive composition is an aqueous dispersion of an adhesive resinous composition comprising polyvinyl acetate and contains a plasticizer therefor.

References Cited in the file of this patent UNITED STATES PATENTS 1,408,746 Kick Mar. 7, 1922 1,451,145 Eckstein Apr. 10, 1923 2,031,035 Dreymann Feb. 18, 1936 2,099,301 Hamersley Nov. 16, 1937 2,341,379 Householder et a1. Feb. 8, 1944 2,341,845 Mark Feb. 15, 1944 2,362,373 Hallman Nov. 7, 1944 2,470,465 Broeren et a1 May 17, 1949 2,824,013 Lang Feb. 18, 1958 

1. THE METHOD COMPRISING CONVERTING A FIBERBOARD SHEET TO A CARTON TO BE CLOSED AND SEALED BY FLAPS AT A CARTON OPENING, SAID FIBERBOARD HAVING A PH IN THE RANGE OF FROM ABOUT 5.2 TO ABOUT 5.7 AND HAVING AN ABSORPTIVE BODY, AND SAID CARTON HAVING FLAPS WITH FACES IN WHICH FIBERS ARE EXPOSED COATING THE ENTIRE CARTON WITH A HYDROPHOBIC COAT OF NORMALLY SOLID FUSIBLE MOISTURE-BARRIER MATERIAL HAVING A MELTING POINT AT AN ELEVATED TEMPERATURE BELOW THE BOILING POINT OF WATER, APPLYING AN AQUEOUS ADHESIVE COMPOSITION ACTIVATABLE TO ADHESIVENESS BY LOSS OF WATER THEREFROM TO SAID FLAPS WHEREBY SAID ADHESIVE COMPOSITION WILL FORM AN INTERFACE BETWEEN PAIRS OF SAID FLAPS WHEN SAID FLAPS ARE CLOSED, CLOSING SAID FLAPS TO SEALING POSITION, HEATING THE SO-POSITIONED FLAPS BY APPLYING HEAT AT A TEMPERATURE ABOVE THE BOILING POINT OF WATER AND THEREBY MELTING AND ABSORBING SUBSTANTIALLY ALL OF THE MOISTURE-BARRIER MATERIAL AND EXPOSING FIBERS OF THE FLAPS TO THE INTERFACIAL AQUEOUS ADHESIVE MATERIAL, AND THEREBY REMOVING WATER FROM THE ADHESIVE MATERIAL AND BONDING FLAP-TO-FLAP BY A FIBER-TO-FIBER BOND. 